xv. I CETACEA. 251 



across the whole of the humeral trochlear surface (see Fig. 

 84). The ulna is a complete and distinct bone in the Pi 

 Hippopotamus, Tapir, and Rhinoceros. In the Ruminants 

 it is more or less rudimentary and fixed behind the radius. 

 In the Camel the two bones become completely coalesced. 

 In the Horse the olecranon and upper part of the shaft 

 alone remain, firmly ankylosed to the radius. 



In the Proboscidea the humerus is remarkable for the 

 great development of the supinator ridge. The ulna and 1 

 radius are quite distinct, and permanently crossed. The* 

 upper end of the latter is small, while the ulna not only 

 contributes the principal part of the articular surface for the / 

 humerus, but has its lower end actually larger than that of; 

 the radius, a condition almost unique among Mammals. 



In Hyrax the humerus is straight/ with, a very prominent 

 outer tuberosity, moderate deltoid ridge, rather compressed 

 inferior extremity, large supratrochlear, but no supracon- 

 dylar, perforation. The ulna and radius are complete and 

 subequal, often ankylosing together in old animals. 



In the Cetacea, the bones of the arm and fore-arm are 

 usually very short, broad, and simple in their characters 

 (sec Fig 99). The humerus has a large globular head, which 

 moves freely in the glenoid cavity of the scapula, the tubero- 

 sities are fused into one, the bicipital groove being absent ; 

 the lower end is broad and flattened, and its inferior surface 

 is divided into two nearly equal flat surfaces placed side by 

 side (one external, the other internal), and meeting at a very 

 obtuse angle. The equally flat upper surfaces of the radius 

 and ulna are applied to these and so united that scarcely 

 any motion is permitted between them, and often in old 

 animals ankylosis takes place at the joint. 



I he ulna and radius are parallel to each other without 



